1. Field of the Invention
The present invention is directed to a surface-active wave filter (SAW) and, specifically, to a SAW filter of the reactance filter type with improved stop band suppression as well as to a method for the optimization of the stop band suppression.
2. Description of the Related Art
Reactance filters are known from classical filter technology. When SAW resonators are employed for the individual resonators instead of discrete elements (inductors and capacitors), then this is called a SAW filter according to the reactance filter type.
Given SAW filters of the reactance filter type, SAW resonators are employed as impedance elements. FIG. 1 shows the schematic structure of a known resonator. It comprises metallic structures on the surface of a piezo-electric substrate and has a terminal pair 1-1 and 1-2 to which an interdigital transducer 1-4 is connected for the transformation of electrical energy into acoustic energy. A reflector1-3 and 1-5 is respectively arranged at both sides of the interdigital transducer 1-4 along the acoustic axis in order to prevent the acoustic energy from escaping.
FIG. 2, on the left, shows the equivalent circuit diagram for a SAW resonator R and shows the symbol employed for the resonator at the right. A series resonant circuit composed of dynamic inductance L1, dynamic capacitor C1 and dynamic resistor R1 (when taking losses into consideration) is located in the first branch of the parallel circuit, and the static capacitor C0 of the interdigital transducer is located in the second branch. The series resonant circuit reflects the behavior of the resonator in the resonance case, i.e., in the range of the resonant frequency fr. The static capacity reflects the behavior in the frequency ranges f less than  less than fr and f greater than  greater than fr. The dynamic capacitor C1 is proportional to the static capacitor C0 of the interdigital transducer:
C1-C0 xe2x80x83xe2x80x83(1.1) 
A resonator has a resonant frequency fr and an anti-resonant frequency fa. The following applies to the resonant frequency fr:                               f          r                =                              1                          2              ⁢              π                                ⁢                                                    L                1                            *                              C                1                                                                        (        1.2        )            
The following applies for the anti-resonant frequency fa of a resonator:                               f          a                =                              f            r                    *                                    1              +                                                C                  1                                                  C                  0                                                                                        (        1.3        )            
The basic unit of a SAW reactance filter is a xe2x80x9cbasic elementxe2x80x9d as shown in FIG. 3. It is composed of a first resonator R1 with resonant frequency frp and appertaining anti-resonant frequency fap in the parallel branch and of a second resonator R2 with resonant frequency frs and appertaining anti-resonant frequency fas in the serial branch. The frequency curve of the admittance Yp of the resonator R1 in the parallel branch and the frequency curve of the impedance Zs of the resonator R2 in the serial branch are shown in FIG. 4. For producing a band-pass filter with the middle frequency f0, the resonant frequencies of the two resonators have the following relationship:
fap≈frs≈f0 xe2x80x83xe2x80x83(1.4) 
Each basic element is fundamentally viewed as a two-port element with the terminals 3-1 or 3-2 of port 1 and the terminal 3-3 or 3-4 of port 2 (see FIG. 3). At the same time, the terminal 3-1 is the input and the terminal 3-3 is the output of the series resonator. The input of the parallel resonator is connected to the terminal 3-1. The terminals 3-2 and 3-4 represent the reference ground for an asymmetrical operation. The output 3-4 of the parallel resonator that faces toward the reference ground is referred below as the output side or ground side of the parallel resonator. The inductance Lser that lies between the output side of the parallel resonator and the reference ground reflects the connection to the housing ground in the real structure.
The selection level of the SAW filter according to the reactance filter type is defined, first, by the relationship C0p/C0s of static capacitor C0p in the parallel branch and static capacitor C0s in the series branch and is defined, second, by the number of basic elements connected following one another (in a cascaded manner, i.e., in series).
The basic elements when they are connected in series are usually circuit-adapted, i.e., they are respectively mirrored. FIG. 5 and FIG. 6 show two examples of a reactance filter in which respectively two basic elements are cascaded. The output impedance 5-1 (Zout) or 6-1 (Zin) of the first basic element is equal to the input impedance 5-2 or 6-2 of the second basic element, resulting in a minimization of losses due to mismatching. Many structures are possible or known for reactance filters with respect to the number and arrangement of the basic elements.
Resonators of the same type (series resonator or parallel resonator) lying immediately behind one another can also be respectively combined to form one in which the overall capacity remains the same. The interconnection of a filter according to FIG. 7 corresponds in effect to that of a filter according to FIG. 8.
FIGS. 9 and 10 show the typical, actual structure of a SAW filter on a piezoelectric substrate 9-1 in a ceramic housing 9-0 and the typical connecting technique with bond wires 9-8 through 9-12 or 10-9.
At the output side 9-15 through 9-17, the parallel resonators R1, R3 and R5 are connected to the housing ground pads 9-4, 9-5 and 9-7 via bond wires 9-9, 9-10 and 9-12.
As a result of the typical structuring technique (see FIG. 9 and FIG. 10), series inductances between, for example, the output side 9-17 of the parallel resonator R5 on the substrate (chip) 9-1 and the ground 10-5 next to the outer housing pin 9-4 are obtained given the connection of the parallel branches to ground. These essentially include the inductive part of the stripline on the chip, the inductance of the bond connection 9-9 and that of the housing lead-through 10-3.
These series inductances influence the behavior of the filter both in the passband as well as in the stop band. f less than  less than f0 applies for the pass band. The resonant frequency and, thus, the bandwidth of a resonator can, as known, be modified by an external wiring belonging to the resonator. An inductance serially with the resonator increases the effective dynamic inductance, resulting in a drop in the resonant frequency fr. Since the anti-resonant frequency fa is shifted to only a very slight extent, the bandwidth xcex94=fa-fr of a resonator is increased with the serial inductance. The bandwidth of the SAW filter is also increased in the case of a parallel resonator.
f less than  less than f0 and f greater than  greater than f0 applies for the stop band. Here, the equivalent circuit diagram of a resonator is reduced to its static capacitance C0 since the series resonant circuit is extremely high-impedance beyond f0 and corresponds to a no-load condition. An inductance Lser connected serially with the resonator produces the series resonant circuit shown in FIG. 11b having a resonant frequency.                               f          pol                =                              1            /            2                    ⁢          π          ⁢                                                    L                ser                            *                              C                0                                                                        (        1.5        )            
In the case of an inductance connected serially with a parallel resonator, this means that the energy of the filter can flow off directly to ground given the frequency fpol; a xe2x80x9cpole pointxe2x80x9d thus forms in the filter curve, i.e., an increased suppression in the stop band. A plurality of pole points in the stop band corresponds to the plurality of parallel branches with series inductance. Pole points fpol1 and fpol2 that can be distinguished from one another in terms of frequency derive only given different products II1=Lser1*C01 and II2=Lser2*C02. When the products are identical, then the pole points lie at the same frequency; a double pole point fpol=fpol1=fpol2 is obtained with a higher suppression than that of a simple single pole point.
FIG. 11b shows the attenuation behavior of a resonator in the parallel branch to which an inductance Lser is serially connected to the output side of the parallel resonator. As in FIG. 11b, the series resonant circuit of the resonator (whose resonant frequency frp=F0) was removed in order to illustrate the pole point. What typically applies for the frequency of the pole point fpol is fpol greater than f0, where f0 is equal to the resonant frequency of the filter. A high attenuation is then obtained for the pole point.
Saw filters of the reactance filter type are mainly employed as RF filters in the mobile radio telephone field since they exhibit extremely low losses in the pass band. As and RF filter in the mobile radio telephone field, the SAW filter of the reactance filter type must, over and above this, suppress: first, the duplex band (i.e., the reception band given a transmission filter and, conversely, the transmission band given a reception filter); and, second, the signal at the local oscillator frequency (LO) and/or at the image frequency in order to prevent unwanted mixed products in the telephone.
The local oscillator lies above or below the middle frequency f0 of the filter. The distance from the middle frequency f0 corresponds to the intermediate frequency (ZF) employed for the signal editing. The image frequency has the spacing 2*ZF from the middle frequency f0. Since momentary ZF frequencies in the range of 100-400 MHz are employed, the SAW filterxe2x80x94depending on the applicationxe2x80x94must comprise good attenuation properties of, typically, more than 30 dB in the range f0xc2x1100-800 MHz. In the most frequent instances, the local oscillator lies above the middle frequency f0.
There are various possibilities for achieving an adequate attenuation in the range of the LO frequency and/or image frequency. According to a first possibility, the general selection level may be made correspondingly high (the minimum attenuation below the pass band given approximately f0/2 is valid as a criterion for this). The great disadvantage in this situation is, however, that the losses in the pass band also increase with an increasing selection level. This is unacceptable for the signal processing in the telephone in most cases.
The second possibility is based on the previously mentioned fact that an inductance present for the traditional structuring technique generates a pole point serially with a parallel resonator that lies exactly at the LO or image frequency. Given the great spectrum of ZF frequencies employed, a possibility must be established in this case in order to vary the generated pole point over a range of approximately 700 MHz.
Since the static capacitance C0p in the parallel branch is the determining factor for the filter performance (passband, matching and selection level), the pole point can only be varied to an extremely slight degree with given filter demands such that the position of pole points in the stop band also simultaneously changes.
Likewise, the degree of freedom for the size of the inductance serially between output side of the parallel resonator and ground is limited. The inductive part of the stripline on the chip can be varied to only a limited extent, due to the necessity for miniaturization as well as for cost reasons, the chips that are employed are becoming smaller and smaller. The length and the inductance of the associated bond connection can likewise hardly to be varied any more with any housing in the course of the progressing trend towards miniaturization. Moreover, the inductance that derives from the housing lead-through a fixed for a given housing.
This second possibility is thus also not established anymore to an adequate extent for SAW filters according to the reactance filter type in housings that have been miniaturized further. This possibility is no longer established to an adequate degree in order to assure the LO and/or image suppression via suitably placed pole points over a great frequency range from f0 plus 100-800 MHz.
It is impossible to generate pole points at relatively low frequencies (i.e., in the range of 100 MHz above the middle frequency f0), particularly given the future connection technology of xe2x80x9cflip-chip-techniquexe2x80x9d in which bump connections are employed instead of the bond wires, since the inductances present given this structuring technique serially to the output side of a parallel resonator are too small (see Equation 1.5), and the static capacitances of the parallel branches can likewise not be selected great enough because of the required self-matching to 50 xcexa9.
It is therefore an object of the present invention to provide a way of designing a filter such that an improved stop band suppression can be obtained for specific LO frequencies and image frequencies over a possible range from 100 through 800 MHz next to the middle frequency. In particular, a way is provided for shifting pole points of a reactance filter into a desired region close to the middle frequency f0 without greatly influencing the remaining filter behavior.
This object is achieved by a surface-active wave (SAW) filter of a reactance filter type, comprising a first SAW resonator in a parallel branch of the filter that has a static capacitance; a further first SAW resonator in a further parallel branch of the filter that has a static capacitance; a second SAW resonator in a serial branch of the filter that has a static capacitance; at least one basic element fashioned on a piezoelectric substrate, the basic element comprising the first SAW resonator and the second SAW resonator; an electrical connection of ground sides of the first SAW resonator and of the further first SAW resonator (collectively, two first resonators), the electrical connection of the ground sides configured to be made before bonding to a housing that contains the filter; and a bump connection on a housing link of the two electrically connected ground sides of the two first resonators; wherein at least the static capacitance of the first SAW resonator and the static capacitance of the further first saw resonator differ from one another.
This object is also achieved by a method for manufacturing this SAW filter, comprising shifting a pole point in the SAW filter; raising or lowering the static capacitance of at least one of the first SAW resonator and the further first SAW resonator; and raising or lowering a static capacitance of one or more further, non-coupled first resonators such that an overall sum of the static capacitances of all parallel resonators remains identical.
A coupling of the parallel branches is inventively produced as a result of a connection of the ground-side output sides of the parallel branches respectively comprising a resonator on the chip. This permits a greatly modifiable frequency positioning of the appertaining pole point (also referred to as xe2x80x9ccoupled pole pointxe2x80x9d below).
As a result, it is possible to produce a SAW filter that comprises pole points at lower frequencies that could be achieved by the previous, serial interconnection of the parallel branches with existing, structure-conditioned inductances according to Equation (1.5). It is also possible to shift one or more pole points in a given filter over a broader frequency range than was previously possible in a given filter. With the invention, thus, a pole point can be generated at exactly the frequency at which a high selection is required, for example, at an arbitrary LO or image frequency.
Such demands for the suppression of signals at the local oscillator frequency (LO suppression) and/or at the image frequency (image suppression) can thus still be satisfied in extremely small housing having very low structure-conditioned inductances. One or more pole points can be shifted to a desired frequency given an established bond inductance, conduct inductance or housing lead-through inductance without this requiring an increase in the serial inductance. Additionally, of course, the serial inductance can also be increased.
Moreover, the plurality of ground connections that are offered can be set independently of the plurality of parallel branches employed, this leading to a lower space requirement. It is precisely in view of new connecting technologies (bump connections instead of bond connections) and new housing technologies that the embodiments according to the invention represent the sole possibility for achieving the previously mentioned selection demands in miniaturized housings.
The principle for shifting the pole points according to the present invention is explained in greater detail below on the basis of exemplary embodiments and the appertaining figures. The following, concrete embodiments are examples of the employment in a SAW filter of the reactance filter type.